1. Field of the Invention
The present invention relates in general to modems and in particular to circuitry for impedance matching between a modem port and a telephone line and systems and methods using the same.
2. Description of the Related Art
With the advent of the Internet and the World Wide Web, most consumers now demand the inclusion of an integrated high speed modem when purchasing a new personal computer. Moreover, consumers are increasingly demanding that these modems not only support the high speed telephonic exchange of computer generated data, but also of voice and facsimile data as well. Additionally, features such as support for echo-canceled digital speaker phones and upgradability to meet the requirements of the newer Digital Simultaneous Voice and Data (DSVD) and video conferencing standards are often also desirable. Thus, the designer of modem chipsets is typically faced with a number of significant technical challenges.
One particular challenge is the design of the interface of the typical 4-wire differential modem port to the full-duplex 2-wire common telephone lines. Either a duplexor or a 2-wire to 4-wire hybrid converter is required to separate the transmit and receive signals at the interface. In the case of a hybrid converter, a resistor-capacitor network is carefully matched with the telephone line impedance reflected across a transformer. In the ideal case, the crossover of transmit signals into the receive path is eliminated by signal cancellation. However, resistors are difficult to match against each other and the telephone line impedance may vary from connection to connection. As a result, transmitter to receiver signal cancellation suffers and a significant amount of transmit signal can be present at the input to the modem analog to digital converter.
Given the demand for increased modem performance, the need has arisen for circuits, systems and methods for interfacing a modem with a telephone line.
According to the principles of the present intention, an interface is disclosed for coupling a modem port with a transmission line. A hybrid converter is included which has a variable gain amplifier coupling a transmit path of a first differential polarity and a received path of a second differential polarity.
The present inventive principles provide for the cancellation of transmit signals in the received path of the embodying modem by allowing the modem interface impedance to be adjusted with respect to the telephone line impedance. As a result, the large dynamic range of the received path can be preserved notwithstanding significant variations in telephone line impedance.